1. Field of the Invention
The invention relates to a process for the preparation of a liquid fuel suitable for burning using a pressure-type atomizer, using a hydrocarbon feed relatively low in hydrogen content and the combustion of such fuel.
2. Relevant Art
It is difficult to burn low quality fuels, e.g., resids, efficiently with minimum impact on the environment, i.e., complete combustion with low soot formation and low particulate emissions.
Low quality, heavy fuels such as resids are used to fuel power plants, ship boilers, and some refinery process units. These fuels are deficient in hydrogen and prone to form soot, leading to black smoke and particulate emissions. It is known that such fuels can be upgraded by hydrogenation. Hydrogenation of heavy resids is a very expensive refinery process, because of the high pressure required and high hydrogen consumption. As an alternative to the chemical addition of hydrogen to residual fuels to improve their combustion quality, the art has sought alternative ways of improving these fuels.
A significant improvement in the efficiency of burning heavy fuels can be achieved if a pressure-type atomizer is used. These atomizers result in the formation of minute droplets of liquid which leave the atomizer to enter a combustion chamber. Usually the feed is heated before reaching the atomizer, so that the viscosity of the feed is low enough for the feed to be atomized. Typically, atomizers work with feeds with a viscosity, measured at the temperature encountered in the atomizer, below 100 CS, and preferably below about 20 CS.
The art has recognized that pressure atomization is not a complete solution, and has attempted to overcome the shortcomings of this method by adding various substances to the feed which promote vaporization and/or reduce the viscosity of the feed. Typical of this work is that described in U.S. Pat. No. 3,672,853, the entire contents of which is incorporated herein by reference. The patentees taught dissolving carbon dioxide, hydrogen or water vapor in the feed in an amount not exceeding 95 percent of the maximum amount that can dissolve at the temperature and pressure just before atomization. The dissolved materials all reduced the viscosity of the feed. At 280.degree. C., 95 kg/cm.sup.2 CO.sub.2 pressure, the viscosity of a heavy feed was reduced from 1670 CP to 820 CP, with the dissolution of CO.sub.2.
In a similar test, with hydrogen addition, more than twice this pressure was used. A feed with an initial viscosity at 280.degree. C. of 1580 CP was reduced to 1350 CP.
Another example in this reference showed equilibrating the same feed used in Example 1 with steam at a steam pressure of 60 kg/cm.sup.2 which resulted in reducing the viscosity by approximately a factor of 2 (based on linear extrapolation).
At the 280.degree. C. temperature, hydrogen was by far the least effective additive regards viscosity reduction, and also the least effective regards the severity of conditions needed to dissolve the gas, hydrogen in this case, in the heavy liquid feed. The effectiveness of the hydrogen as a viscosity reducer tended to increase, relative to CO.sub.2 and H.sub.2 O, at higher temperatures.
The patentee did not report any results of actual burning experiments using any of these additives, namely CO.sub.2, H.sub.2, or H.sub.2 O. Apparently the inventors sought better atomization of fuel, rather than reduced sooting tendency. One advantage mentioned for the process was permitting burning heavy fuels to form soot, and recycle the soot back to the feed. Soot recycle increases feed viscosity, but the viscosity would then be lowered by the practice of their invention.
Unfortunately, none of the solutions to minimizing the sooting tendency of heavy, halogen deficient fuels suggested by U.S. Pat. No. 3,672,853 was completely satisfactory.
Use of atomizing steam improves combustion of heavy fuels, but sooting was still a problem.
CO.sub.2 is a moderately expensive, and corrosive, commodity. Addition of CO.sub.2 reduces the viscosity of a heavy feed, but does nothing towards reducing the sooting tendency of a heavy feed, although better atomization may reduce soot formation.
Dissolution of hydrogen in a heavy feed reduces the sooting tendency of the feed, but hydrogen gas is a very valuable commodity in a refinery. It would be expensive, in terms of compression costs if nothing else, to achieve sufficiently high hydrogen partial pressures to significantly reduce the viscosity and (inherently) the sooting tendencies of a heavy fuel. Many existing fuel distribution systems, atomizers, pumps, etc. could not tolerate the high pressures required.
Some work was done in Japanese Pat. No. 78491 towards reducing the smoking tendency of diesel fuels. Basically an LPG, or liquified petroleum gas stream consisting primarily of propane and butane is dissolved in a tank of diesel fuel to reduce the sooting tendency.
Diesel fuel is a good quality fuel if you just want to burn it in a boiler or process heater. No elaborate steam atomization, etc. is required to cleanly burn this premium liquid fuel. Improving the burning characteristics of diesel fuel by adding valuable materials such as propane and butanes represents very poor use of these materials, but such a fuel would have little sooting tendency.
Other hydrogen deficient fuels range from very light materials, such as acetylene, to heavier liquids, such as highly aromatic fuels such as toluene or liquids derived from coal.
We discovered a way to improve the burning tendencies of these hydrogen deficient light and heavy fuels, without resorting to the use of hydrogen, and the high cost both of the raw material and the compression costs associated with dissolving the hydrogen in the fuel. We discovered an efficient and economical way to upgrade hydrogen deficient fuels, at least as far as reducing their sooting tendencies, by addition of light hydrocarbon gases to the fuel. Some viscosity reduction, and improved atomization of heavy fuels will also be achieved.